WASHINGTON -- Lois Lerner, the head of the IRS office under fire for improperly targeting conservative groups, was replaced by the agency's acting commissioner on Thursday, a day after she insisted she had done nothing wrong and refused to answer questions before a congressional committee.

Lerner, who has been with the IRS for 12 years, was head of the IRS office of exempt organizations, the unit that is tasked with policing charities and other nonprofits that get tax-exempt status. She has been placed on administrative leave, according to a congressional source who asked not to be identified.

An inspector general’s audit found that staff in a Cincinnati field office used terms such as “tea party” and “patriots” to select applications for extra scrutiny. Congress is investigating the IRS' actions, and the FBI has begun a criminal investigation.

In an email to employees, IRS Acting Commissioner Danny Werfel said he had chosen Ken Corbin as Lerner’s replacement. Corbin, a 27-year agency employee, was a deputy director in the IRS division that processes tax returns. Werfel said Corbin’s experience and strong management background made him “an ideal choice to help lead the exempt organizations area through this difficult period.”

President Obama named Werfel, a career budget analyst, to take over the agency after the forced departure of its prior acting director. He started Wednesday.

Lerner’s departure was first reported by the National Review. The IRS and Lerner’s attorney did not immediately respond to requests for comment.

On Thursday, the top two senators on the Senate Permanent Subcommittee on Investigations sent a letter to Werfel urging that Lerner be suspended.

“We believe that the immediate removal of Ms. Lerner from office would be a vital step in helping to restore public trust in the agency,” said the letter, signed by John McCain (R-Ariz.) and Carl Levin (D-Mich.)

The audit found that Lerner ordered changes in the screening when she first found out about it, in June 2011. But the audit also blamed the errors on failures of IRS management.

The chairman of the House oversight committee, Rep. Darrell Issa (R-Vista), said that committee lawyers believe that Lerner waived her 5th Amendment privilege when she gave a statement. She may be recalled for more questions, he said.